There is a great deal of what you might call “raw material” out in space, and much of it is pretty cold and hard to see. The formation of the objects we can see in space, including stars, nebulae, and the objects that are actually collections of other objects (e.g., star clusters and galaxies), started with these raw materials. Gas makes up roughly 99% of the raw material found in space. The word gas, as used in astronomy, refers to loose collections of atoms or molecules, often found in giant “clouds” among the stars (known as nebulae). The most common kind of gas found in space is made up of hydrogen atoms, which glow with a characteristic red color when “excited,” i.e., energized by heat or radiation. (You can see the red glow of hydrogen on many of the cards in the Cosmic Decoders Card Set.) The next most common gas in space is made up of helium atoms. Other gases are also common, such as nitrogen and oxygen, although they are much rarer in space than hydrogen and helium.

Another common raw material in space is dust. The term dust in astronomy simply means individual grains of solid material (denser collections of atoms and molecules). Dust in space is typically found in the same clouds among the stars where gas is found. Astronomers believe that the dust in space is not that different from what you find under your bed when you are too distracted by your love of astronomy to clean up properly.

A nebula is a cloud of gas and dust in space. A nebula can be “lit up” by the energy of one or more nearby stars (including stars inside the nebula). In this state, the cloud of gas and dust becomes visible to us (although most are only visible with the aid of telescopes). The plural of nebula is nebulae (pronounced neb’ u lee). Nebulae are found in galaxies. Some nebulae are small while others are huge. Some can contain thousands of stars while others surround just a single star. Some nebulae are “star nurseries,” otherwise known as star birth nebulae, where new stars are formed. Other types of nebulae, called star death nebulae, are produced when a star loses its raw material at the end of its life.

A star birth nebula (or star formation region) is a cloud of gas and dust in which new stars are being formed from the raw material of the cloud. In places where the cloud has become compressed (where the gas and dust are very close together), the gas and dust start “clumping.” At a certain stage, these clumps start heating up in the center, and when that happens, star formation begins. After many years of clumping and heating, a star will get so hot in the middle (over 10 million degrees Centigrade) that it will begin to make its own energy – it starts to shine. In many of the star forming regions depicted on the Cosmic Decoders Cards, new stars have already begun to shine, and their energy makes the clouds of gas and dust around them glow with beautiful colors.

When a star nears the end of its existence, astronomers say it begins to “die.” One characteristic of dying stars is that they lose the raw material that helped form them. A star death nebula is thus formed from the raw material expelled from a dying star. The way in which the raw material is expelled determines the type of resulting star death nebula. There are two types of star death nebulae: planetary nebulae and supernova remnants.

A planetary nebula has nothing to do with planets. Instead, it is created when a dying star loses material all around it. This material leaves the star in one or more “last gasps” before the star dies, and creates a sort of “shell” (made mostly of hot gas) around the dead star. Sometimes the gas comes off pretty much uniformly, giving the shell a ball or donut shape (as seen in the Retina Nebula from the Cosmic Decoders Card Set). At other times, old material that is already around the star closes off some “avenues of escape” for newly lost material (which creates yet its own shell). Then the shape of the nebula can be complicated, as you can see in the Saturn Nebula from the Cosmic Decoders Card Set.

We can see the glowing shells of gas from planetary nebula because the atoms within are “excited” (energized by heat or radiation) by the energy of the hot dead star, or star “corpse,” in the middle. The dead star is called a white dwarf because it is now much smaller and white-hot. Planetary nebulae and white dwarfs form when relatively smaller stars, like our Sun, die. However, when larger stars die, another type of star death nebula, called a supernova remnant, is formed.

A supernova remnantconsists of the gas and dust left over from the explosion of a star at the end of its life. Instead of “quietly” letting off some material (as the smaller stars mostly do), the really big stars blow up when they die, scattering their fragments into space at great speed. Years after the explosion, we see these fragments, or remains, of gas and dust spread out over a great distance. We can see them because sometimes a star corpse still survives (known as a pulsar, a rapidly whirling object) and it provides energy to light up the gas and dust remains (as in the Crab Nebula). In other supernova remnants, the exploded material collides with other material in space, and it is the energy of these collisions that causes the gas to glow.

A star cluster is a group of stars that are born together and move through space together. In general, stars in such a cluster are closer to each other than other stars’ neighbor stars tend to be. Star clusters can include as few as a dozen stars or as many as a million stars. Star clusters are found in galaxies.

An open star cluster is a relatively small star cluster group with anywhere from a dozen to a thousand stars. Such star clusters tend to be much more loose and spread out than the other type of star cluster — the globular star cluster. A globular star cluster is a large, tightly bound group of stars, which typically has a hundred thousand or more stars.

A galaxy is a collection of many millions or billions of stars, nebulae, and other cosmic objects. Our Sun and its planets are part of one such collection, called the Milky Way Galaxy. One of the great discoveries in astronomy during the 20th century was that there are billions of other galaxies (or “islands” of stars and other cosmic objects) throughout the universe.

Galaxies tend to be “social,” in that they come in groups. Our Milky Way Galaxy, for example, is part of a loose group of several dozen galaxies that we call The Local Group. Some large groups can contain thousands and thousands of galaxies. In a group, collisions among galaxies can occur over time. When two galaxies collide, their stars are mostly too far apart to suffer actual collisions. But the nebulae of raw material (gas and dust) in them can come together and get compressed (squeezed together). This compression often leads to the formation of many new stars. In colliding galaxies, we not only see “bursts” of new star formation, but we also see the overall shapes of the galaxies get distorted, sometimes resulting in what are called warped disk galaxies (http://antwrp.gsfc.nasa.gov/apod/ap010803.html) and polar ring galaxies (http://antwrp.gsfc.nasa.gov/apod/ap990510.html). Several of the galaxies depicted on the Cosmic Decoders Cards show the results of colliding galaxies.

A spiral galaxy is a basically flat galaxy with a disk shape, something like a Frisbee® with a rounded bulge in the center. In such galaxies, the stars and other objects that make up the disk are typically organized into a multiple-armed spiral structure, like a pinwheel. Spirals generally have quite a bit of gas and dust in addition to stars. Many spiral galaxies have a bar of stars and raw material across their centers, and hence are called barred spirals.

An elliptical galaxy has an overall curved elongated shape like a football or a blimp. Some are much more round like a basketball. Such galaxies typically have very little raw material (gas and dust) and contain more older stars. Ellipticals come in a variety of sizes from small, loosely packed dwarf ellipticals, all the way to giant ellipticals that can contain a thousand billion stars or more!

An irregular galaxy is a galaxy whose appearance is more complicated than either a spiral or an elliptical. Irregulars come in a bewildering variety of shapes, which are probably the results of collisions of two or more galaxies. As galaxies collide (see above for more), their mutual gravity can distort the stars, gas, and dust in them into many different shapes.

All the magnificent Hubble Space Telescope images are here, with captions — many with detailed background information and explanations of the pictures. You can check the latest images, browse the ones their staff consider to be Hubble’s “greatest hits,” or search for objects of interest to you. Since the pictures are not under copyright, they can all be downloaded in various formats completely free.

The National Optical Astronomy Observatory (NOAO) includes a number of major telescopes in the United States and in Chile. Some of the best images from NOAO’s instruments are collected and organized at this site and they too are free for private use.

Here astronomers Robert Nemiroff and Jerry Bonell feature one relatively new celestial image each day with a brief non-technical caption. Over the years, some of the best astronomical images have been featured and an index is available on site.

This growing album contains images from the large telescopes in the southern hemisphere run by a consortium of European countries. With the building of the Very Large Telescope in Chile, which will be the biggest telescope in the world, there is an increasing number of important new images on this site.

This site provides a marvelous library of images (focusing on nebulae and galaxies) taken using large telescopes in Australia. Many are by David Malin, who is acknowledged to be one of the finest astronomical photographers of our time. Includes captions and ordering information. (Note these images are copyrighted!)

This site collects infra-red images from space missions and telescopes on the ground and also offers an intriguing set of comparisons showing images at visible and invisible wavelengths. Note: To get to the picture collections, wait for the top page to load and then click on “Image Galleries.”

Here you can find pictures constructed from radio waves, collected by a variety of U.S. radio telescopes. The universe as seen with “radio eyes” is quite different from the universe visible light reveals to us.

The Chandra x-ray telescope (in orbit around Earth) has been sending back a wide range of images of what deep space objects would look like if seen with “x-ray sensitive” eyes. The images are arranged by category for easy sorting.

If you have a question about astronomy not answered on our web site, here are some sites where astronomers are answering questions from the public. Before you ask your questions at a particular site, check out their archives, where former questions from others and the answers the scientists provided are available for all. Chances are someone has asked your question already, and you can get your answer instantly.

Questions for and answers by the staff of the National Solar Observatory, with a particular focus on the Sun and the Solar System. Note: A much more specialized site, just for questions about the Sun, has been set up by the team for the SOHO satellite at: http://soho.nascom.nasa.gov/explore/drsoho.html

Astronomer Phil Plait started this site to deal with astronomical mistakes and misconceptions in the media, but has expanded it to give brief answers to questions on a variety of astronomical topics. Contains a table of contents, organized by topic.

An atom is the smallest “piece” of an element that still has the properties of that element. For example, an atom of gold is the smallest piece of gold that still acts like gold — divide that atom any further and the particles inside are no longer gold. Atoms are much too small to see or feel with our human senses, but evidence from scientific instruments and studies clearly indicates that they are at the heart of all objects and matter in the universe. When many atoms come together, atoms tend to form molecules.

A moleculeis a combination of two or more atoms bonded together. For example, a molecule of water (designated by the symbol H2O) consists of two hydrogen atoms and an oxygen atom which are in a “relationship” — held together by an electric attraction.

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